Sendmail and its aversion to upper case

I know lower case rules the Unix world. But at my home for some weird issue with samba shares I had to create a user with upper case "Bob". And I created my fetchmailrc with a very simple no nonsense configuration.

poll mail.X.org proto pop3
user bobnap is Bob here
options keep ssl

When I asked fetchmail to do its job, it did everything correctly and handed over the fetched mail to my local sendmail. Which puked on everything with the following message...

1 message for at mail.X.org (774 octets).
reading message bobnap@mail.X.org:1 of 1 (774 octets) fetchmail: SMTP error: 550 5.1.1 ... User unknown
fetchmail: mail from MAILER-DAEMON@localhost bounced to bobnap@X.org
fetchmail: can't even send to Bob!
not flushed

Intrigued by this I tried to mail myself. The mail ended dead in the "dead.letter".

$ mailx -s "Tot" Bob < /dev/null
Null message body; hope that's ok
$ /home/Bob/dead.letter... Saved message in /home/Bob/dead.letter

I checked and rechecked my sendmail configuration, its aliases etc. Everything seemed alright. Then I logged in as a another user "naper" in my system and tried mailx. Magically everything worked great. Then only I realized the fact that sendmail didn't like the upper case in "Bob" and to confirm that I just edited the passwd files to make it all lower case and sure enough...bob is getting his mails now!

What a strange ugly little devil!